How to limit multiple authentication of single user on 3proxy

I have installed 3Proxy on my ubuntu server and it works fine, I am looking for a solution to limit multiple authentications for a single user. This is since I'm planning to limit the users from giving off their passwords/accounts to each other.

below is my config file:

#
nserver 8.8.8.8
nserver 8.8.4.4

nscache 65536
#Leave default timeout as well:
#
timeouts 1 5 30 60 180 1800 15 60
#
users $/etc/3proxy/.proxyauth

daemon

#
log /var/log/3proxy/3proxy.log
logformat "- +_L%t.%. %N.%p %E %U %C:%c %R:%r %O %I %h %T"
#Compress logs using gzip
#
archiver gz /usr/bin/gzip %F
#store the logs for 30 days
rotate 3

#added authentication caching to make life easier
authcache user 60

# Start new ACLs
#enable strong authorization. To disable authentication, simply change to 'auth none'
auth strong cache

#restrict access for ports via http(s)-proxy and deny access to local interfaces
deny * * 127.0.0.0/8,192.168.1.1
allow * * * 80-88,8080-8088 HTTP
allow * * * 443,8443 HTTPS
# allow SOCKS connection to all ports
# allow * * * 1-65535 CONNECT

# allow SOCKS connection to all ports
# allow * * * 1-65535 CONNECT
#allow * * * 1-65535 HTTPS



#SOCKS
flush
auth strong cache

socks -p6699

#Enable admin web-ui on specified port, only allow connection from loopback interface (127.0/8) & intranet's admin user (10/8)
flush
auth iponly strong cache
allow * * 127.0.0.0/8
allow admin * 10.0.0.0/8